Large and small buildings utilize a tremendous amount of power to heat and cool their respective spaces. Furthermore, this power is usually consumed coincident with the need for heating and cooling within the building. Since most buildings within a utility's service territory will experience peak demands for heating or cooling at the same time, the resources of the utility supplying power are taxed to their limits at these times and are frequently exceeded. This results in brownouts and, in extreme cases, power outages. Clearly, more effective and lower cost alternatives for power demand planning are needed.
In response to this problem, thermal storage systems were developed. These systems, which generally are limited to larger buildings, allow the building's heating or cooling system to be run at off-peak hours. The thermal energy that is produced is saved in the thermal storage system and then used to heat or cool the building during on-peak hours. In other words, thermal storage systems shift power consumption from peak periods to those times when power demand is considerably less as exemplified by Duh, U.S. Pat. No. 5,237,746; Foley, U.S. Pat. No. 4,827,735; Mauer et al., U.S. Pat. No. 5,369,964; MacCracken, U.S. Pat. No. 4,403,645; Schrader, U.S. Pat. No. 4,609,036 and Telkes, U.S. Pat. No. 2,936,741, each of which is incorporated herein by reference.
For a building owner, the only incentive to install a thermal storage system is to reduce utility bills. This can happen since it is often less expensive to purchase power during off-peak periods. Also, by operating heating or cooling periods at lower power but for longer periods of time, as made possible by the use of a thermal storage system, monthly electrical demand charge (i.e., charge for the peak kilowatts drawn during the month) can be reduced. Unfortunately, for a large majority of building owners, the payback for thermal storage systems currently available does not justify the investment.
The most popular thermal storage system for all but the largest buildings are those that build ice on the surface of a plastic heat exchanger. Such systems typically comprise coils or loops of plastic tubing containing a heat transfer fluid that is used to freeze water or other cooling medium as the cold store. The plastic tubing that is used in these systems must have a relatively thick wall (e.g. 0.062") so that (1) the tubing will not collapse during assembly of the heat exchanger, (2) the tubing can be securely joined to the inlet and outlet manifolds, and (3) the tubing will not burst under the pressure of the heat transfer fluid. The relatively thick walls of the plastic tubes increases the weight and the cost of such thermal storage systems.
Thermal energy storage systems that build ice on the walls of a tubular heat exchanger also suffer a performance penalty when discharging the system. When these systems are discharged the ice melts outwardly from the tube. This forms a small annular ring of water that surrounds the tube. Since water has a low thermal conductivity and since the water in this annulus is almost completely stagnant (i.e., there is very little convective heat transfer), heat transfer is impeded between the ice and the heat transfer fluid that flows within the heat exchanger. Because of this, in many applications, a thermal storage system that builds ice on a tubular heat exchanger cannot be discharged at a rate that is sufficient to meet the cooling requirements of a building unless excess storage capacity is installed or mechanical agitation is added. This further increases the costs, weight and space requirements of the thermal storage system.
Thermal energy storage systems that use ice are not currently available in small sizes. Storage modules have capacities typically in the range of from about 60 to 200 ton-hours. They most commonly are applied on commercial buildings where the onpeak period when they must provide cooling is 10 hours or greater.
The typical module sizes for thermal storage systems that make ice are much more than would be needed by a residence (which needs about 25 ton-hours). They are also much too large for a commercial building that needs to defer the operation of its cooling system for only one or two hours. While this is not currently a common need, the implementation of "real time" pricing could create a need. Under "real time" pricing, the market forces of supply and demand set the price for electricity for each hour of the day. Under this pricing scheme, extremely high electric rates could occur for one or two hours each day during the cooling season.
Under "real time" pricing, a small thermal storage system that could provide cooling during the few hours when electric rates were extremely high might provide the most economic benefits. Unfortunately, the ice storage systems that are now on the market are difficult to scale to smaller sizes.
It would therefore be a significant advance in the art of thermal storage systems to provide a compact, light weight, and low cost system that can store heat or cooling during off-peak hours, that can be scaled to smaller sizes, and that can be rapidly discharged to use the stored energy to heat or cool the building during peak demand hours.